Cord fibers to be used in conventional rubber reinforcing cords for use in tires, belts, etc., include rayon, polyamides, polyesters, and the like. Recently, organic fibers such as aramide (aromatic polyamide fibers), and inorganic fibers, such as glass fiber, steel fiber, etc., have also been employed as cord fibers. In particular for use in tires, it is desirable that the rubber reinforcing cords comprise fibrous materials having high strength, high modulus, and lightweight, from the standpoint of controllability when driving, running stability, comfortability to ride on, durability of tires, fuel cost, and the like.
Carbon fibers are a highly promising material for producing excellent rubber reinforcing cords because of their characteristics of much higher specific modulus and specific strength as compared with the aforesaid reinforcing fibers. However, carbon fibers have a disadvantage of poor adhesion to rubber, and many attempts have been made to overcome this disadvantage. For example, there have been proposed a process for producing a reinforcing cord which comprises twisting elastomer-impregnated carbon fibers as disclosed in U.S. Pat. No. 3,648,452; a process which comprises treating carbon fibers with an epoxy compound and then with an adhesive for a rubber, such as a resorcin-formalin-rubber latex type adhesive (hereinafter referred to as RFL) as disclosed in Japanese Patent Application (OPI) No. 102678/75 (the term "OPI" as used herein means "unexamined published application"); a process which comprises treating carbon fibers with a first processing bath containing a polyisocyanate and then with a second processing bath containing RFL as disclosed in Japanese Patent Application (OPI) No. 102679/75; and the like.
None of these proposals, however, has been able to attain adhesion to rubber completely sufficient for practical use. Additionally, these conventional processes could not succeed in solving another problem associated with carbon fibers arising from their high modulus of elasticity, i.e., extremely poor resistance to repeated flexing fatigue, such as elongation, compression, and the like.
According to the inventors' studies, the causes of the insufficient performances of the above-described carbon fiber cords are considered to reside in that the elastomer, epoxy resin or polyisocyanate present between carbon fibers and rubber or between carbon fibers and RFL is not bound to the carbon fibers or, if they are bound to the carbon fibers, they do not allow penetration of rubber or RFL into spaces between filaments constituting a carbon fiber bundle, resulting in insufficient adhesion within the carbon fiber bundle.
In order to assist in penetration of RFL into the carbon fiber bundle, use of a water-soluble epoxy resin has been proposed. According to this process, however, since RFL is applied as a water dispersion, the water-soluble epoxy resin not only falls off by dissolving during treatment to reduce the effects, but also contaminates the RFL dispersion.